1. Field of the Invention
The present invention relates to a sheet processing apparatus and an image forming apparatus, and, in particular, to a configuration for aligning subsequent sheets in a waiting state while prior sheets are processed.
2. Description of the Related Art
Conventionally, some image forming apparatuses, such as copying machines, laser beam printers, facsimile apparatuses, and multifunction peripherals having these functions, are provided with sheet processing apparatuses, which perform processing such as binding and sorting sheets with images formed thereon. A widely used type of such sheet processing apparatuses is an apparatus including an intermediate processing tray within the apparatus, and configured to stack a plurality of sheets on this intermediate processing tray to form a sheet bundle and then perform processing such as binding on this sheet bundle.
This kind of sheet processing apparatus requires a certain processing time to perform binding of sheets. Although this processing time partially depends on the image formation speed of the image forming apparatus, generally, the processing time exceeds an interval between discharges of sheets because it is difficult to complete binding processing within the interval between discharges of sheets. Therefore, binding processing requires an interruption of image formation, but this interruption of image formation reduces the productivity.
Therefore, some of conventional sheet processing apparatuses perform buffering processing (waiting processing), according to which, for example, first several sheets of a subsequent sheet bundle is temporarily kept waiting while binding processing is performed on a prior sheet bundle on the intermediate processing tray. Then, upon completion of the binding processing on the prior sheet bundle, the prior sheet bundle is discharged, and after that, the several sheets temporarily kept waiting are conveyed onto the intermediate processing tray in a stacked state. As a result, the sheets can be processed without an interruption of the image formation.
However, if an image forming apparatus can form images at a high speed and discharge sheets at short intervals, it is difficult to complete not only binding processing but also even alignment processing within the interval between discharges of sheets. One technique for solving this problem is a sheet processing apparatus that buffers sheets, which conventionally used to be conveyed to the intermediate processing tray one by one, and conveys a plurality of sheets to the intermediate processing tray at a time, thereby aligning the sheets without interrupting an output from the image forming apparatus (refer to Japanese Patent Application Laid-Open No. 10-194582).
In this conventional sheet processing apparatus, when sheets are processed, while binding processing is performed on a prior sheet bundle on the intermediate processing tray first, a first sheet in a subsequent sheet bundle is wound around a buffer roller to be set in a temporary waiting state. Then, when a second sheet reaches the buffer roller, the rotation of the temporarily stopped buffer roller is restarted, thereby overlapping the first sheet wound around the buffer roller with the second sheet. At this time, the first sheet and the second sheet are overlapping in such a manner that the upstream edge of the second sheet in the conveyance direction is ahead of the upstream edge of the first sheet in the conveyance direction by a predetermined distance in the conveyance direction. Thereafter, a predetermined number of sheets are overlapping in a similar manner so that the upstream edge of the upper sheet in the conveyance direction is ahead of the upstream edge of the lower sheet in the conveyance direction by the predetermined distance in the conveyance direction.
After that, upon completion of the binding processing on the prior sheet bundle and a discharge of the prior sheet bundle, the bundle of the overlapping sheets is discharged onto the intermediate processing tray by discharge rollers, and is transferred to bundle discharge rollers. Then, the trailing edge of the sheet bundle exits the discharge rollers, and at this time, the bundle discharge rollers in pressure contact with each other are rotated in the reverse direction. After that, the bundle discharge rollers are separated, whereby the sheet bundle is released in such a direction that the sheet bundle abuts on a trailing edge stopper of the intermediate processing tray.
On the other hand, this sheet processing apparatus includes a knurled belt for moving the sheet bundle in the direction toward the trailing edge stopper. Then, after the sheet bundle is released, the sheet processing apparatus brings the sheet bundle, in which the plurality of sheets is stacked in such a state that the upstream edges of the sheets are displaced in the conveyance direction, into contact with the trailing edge stopper by the knurled belt one by one sequentially, starting from the lowermost sheet, thereby aligning the discharged sheet bundle in the discharge direction. After that, the sheet bundle is aligned in the width direction perpendicular to the sheet discharge direction on the intermediate processing tray by an alignment plate (not illustrated), thereby completing the sheet alignment processing.
In this way, it is possible to align sheets without slowing down the processing speed of the image forming apparatus that form images at a high speed and discharges sheets at short intervals, by conveying the plurality of sheets onto the intermediate processing tray in a stacked and displaced state.
In such a conventional sheet processing apparatus, a plurality of sheets is stacked in such a manner that the upstream edges of the sheets in the conveyance direction are displaced by substantially equal distances. However, actually, a variation occurs in the distance by which the upstream edges of the sheets in the conveyance direction are displaced due to the influence of, for example, a conveyance error or a variation in the lengths of the sheets in the conveyance direction. If the stacked sheets are discharged with their upstream edges in the conveyance direction displaced by a distance shorter than a predetermined distance, the upstream edge of the upper sheet may reach the knurled belt before the upstream edge of the lower sheet in the conveyance direction is pulled into contact with the trailing edge stopper by the knurled belt. This is because the frictional coefficient between the knurled belt and the sheet is significantly larger than the frictional coefficient between the sheets, so the upper sheet is moved in a sliding contact with the upper surface of the lower sheet to reach the trailing edge stopper before the lower sheet.
Further, the knurled belt is configured to be abutted against the trailing edge stopper while in contact with the upper surface of a sheet. Therefore, if the upper sheet reaches the trailing edge stopper before the lower sheet reaches there, the lower sheet stops before reaching the trailing edge stopper, making excellent sheet alignment difficult.